1. Field of the Invention
The present invention relates to an improved device for countering and tracking a threat and directed counter-measures.
More precisely, it relates to a device for tracking and countering a threat in the form of an infrared homing-head missile (IRHHM).
2. Description of Related Art
FIG. 1 schematically illustrates a known pyrotechnic counter-measure device 3 protecting for example an aircraft 1 from a threat 2. The threat 2 presents in the form of an infrared homing-head missile (IRHHM).
The device 3 is a pyrotechnic decoy generating infrared radiation when fired from the aircraft 1 whenever the latter has detected the threat 2. The infrared radiation generated by the decoy 3, which is more intense than the infrared radiation generated by the aircraft 1, causes deviation of the trajectory of the threat 2 by perturbation of the system guiding homing head of the missile. The threat 2 veers towards the decoy 3 rather than towards the aircraft 1.
Pyrotechnic decoy devices have disadvantages, however.
First, they are cumbersome in design. Also, they represent a fire risk inside the aircraft should they malfunction. In addition, in the event of a false alarm, they considerably compromise the furtive state of the aircraft 1. Finally, they exist in limited quantity only on board the aircraft, and would no longer protect the aircraft which they have been depleted on a mission. Thus they represent consumable devices needing to be replaced regularly and are highly specialised to a certain type of threat.
Consequently, current counter-measure devices are generally infrared illumination jammers.
FIG. 2 schematically illustrates a known directional infrared counter-measure device (DIRCM—Directional InfraRed Counter-Measures).
The aircraft 1 comprises a device for detecting the lift-off of missiles (MWS—Missile Warning System) in the form of a plurality of detectors 6 mounted on the fuselage of the aircraft 1. The detectors 6 detect the launch of a missile 2, tracking the trajectory of the missile and identifying it as a threat to the aircraft 1.
A control device 7 of the launch detection device transmits data on the trajectory of the missile 2 to the control device 8 of a directional counter-measure system.
The control system 8 then initiates the tracking device 4 which tracks the missile and determines its direction in space. The tracking device detects the homing head of the missile 2 by using, for example, the cat's eye effect. The focal plane of the homing head is generally a plane with high reflection coefficient and also cooled, at the same time placed au foyer of the optics for focussing onto this detection plane the signals useful for guiding the missile. The consequence of this in the event of illuminating the homing head, is to send a strong signal towards the light source (such as those sent by the eyes of a cat when illuminated by car headlights, hence the term cat's eyes) and/or in the case of thermal detection to have a cold and extensive object, corresponding to the detection surface of the homing head, an object easily detectable by an infrared camera. This problem is familiar to the specialist under the name of pointed optical detection.
In order to track the missile 2 the tracking device 4 can, if fitted with a thermal camera, detect the cool image produced by the homing head and zero in on it. The tracking device 4 can also emit a light beam 40 in the direction of the homing head of the missile 2 and utilise the signal obtained in return to make this control in direction.
The control device 8 also directs a jamming light beam 50 having an angular opening γ and produced by a source of infrared light 5. The light source 5 generally uses infrared discharge lamps, whereof spectrum has wavelengths covering a wide spectral field, from the visible to the distant infrared. Once tracking is correctly carried out and the missile is in the beam of the light source 5, the infrared illumination 50 is sent towards the homing head of the missile 2 according to a specific sequence to carry out jamming of the missile 2, such that it no longer constitutes a threat to the aircraft 1. The interference signal 50 can be the same as the tracking signal 40.
The discharge light sources 5 have numerous disadvantages.
Due to the spread of the light source, the concentration of the beam requires a bulky optical device (large pupil, large focus) which is very cumbersome and heavy. Jammers based on discharge lamps are therefore difficult to stow on board aircrafts. Also, the available light power is not very significant, and severely limits the efficacy of such jammers utilising discharge lamps.
Accordingly, with the evolution of technology, it is now possible to obtain infrared laser sources emitting wavelengths of between 3 and 5 micrometers. This evolution in technology brings a significant improvement in tracking and counter-measure devices. In fact, with a laser source, it is possible to substantially boost the intensity of the beam 50 and/or 40 with a source much smaller than a discharge source and closer to the diffraction limit. Due to the fact that the laser source is a coherent source, all the energy of the beam is concentrated on a single wavelength.
There are advantages to boosting the power of the beam on one or more given wavelengths.
A laser beam deposits greater illumination than that of discharge lamps in pupil entry of the homing head of the threat.
Laser energy can be contained in several rays (typically two or three) to be able to effectively light up all types of missile homing heads.
Due to its optical properties (strong luminance, weak divergence), the laser is well adapted to this type of device. To further reduce this divergence and obtain a balanced scope acceptable for functions of active tracking, AD identification and jamming of the DIRCM, optics known as collimation can be utilised. The function of this optics of afocal type is to reduce the divergence according to its enlargement ratio, and at the same time increase the section of the beam in an inverse ratio.
A laser beam can be reflected to a greater degree by the “search head”, such that identification of the missile 2 especially by the modulation it introduces to the reflected laser beam is facilitated. Proper identification of the threat helps send a clear jamming code, that is, a code truly adapted to the type of missile 2. The source is highly directive, and this heightens the general furtive state of the aircraft.
Tracking and counter-measure devices utilising a laser source have disadvantages, however.
Due to the fact of low divergence of the beam laser produced by the laser source (in general less than 1 milliradian), the tracking device 4 must be capable of very precisely tracking the infrared detector of the missile 2.
In addition, the coherent characteristic of the laser sources imposes the use of a beam shaping, orientation and stabilisation device which does not generate interference which might have harmful consequences on the general functioning of the system and particularly on the jamming efficacy of the homing head of the missile.
It is therefore very difficult to design a tracking and counter-measure device which can rapidly and simply follow the missile in the event of using an infrared laser source.